Covalent Organic Framework Nanoplates Enable Solution-Processed Crystalline Nanofilms for Photoelectrochemical Hydrogen Evolution

As covalent organic frameworks (COFs) are coming of age, the lack of effective approaches to achieve crystalline and centimeter-scale-homogeneous COF films remains a significant bottleneck toward advancing the application of COFs in optoelectronic devices. Here, we present the synthesis of colloidal COF nanoplates, with lateral sizes of ∼200 nm and average heights of 35 nm, and their utilization as photocathodes for solar hydrogen evolution. The resulting COF nanoplate colloid exhibits a unimodal particle-size distribution and an exceptional colloidal stability without showing agglomeration after storage for 10 months and enables smooth, homogeneous, and thickness-tunable COF nanofilms via spin coating. Photoelectrodes comprising COF nanofilms were fabricated for photoelectrochemical (PEC) solar-to-hydrogen conversion. By rationally designing multicomponent photoelectrode architectures including a polymer donor/COF heterojunction and a hole-transport layer, charge recombination in COFs is mitigated, resulting in a significantly increased photocurrent density and an extremely positive onset potential for PEC hydrogen evolution (over +1 V against the reversible hydrogen electrode), among the best of classical semiconductor-based photocathodes. This work thus paves the way toward fabricating solution-processed large-scale COF nanofilms and heterojunction architectures and their use in solar-energy-conversion devices.


Characterization
Atomic force microscopy. Atomic force microscopy (AFM) was performed using Asylum Research MFP-3D, equipped with a micro cantilever (OMCL-AC160TS-R3). TPB-MeOTP-NP and TPB-MeOTP-NS were deposited on SiO 2 /Si wafers by drop cast, and TPB-MeOTP nanofilms were prepared by spin coating as described in the film preparation part.
Nuclear magnetic resonance. 1 H-NMR spectra of the COF linkers and in-situ 1 H-NMR spectra of the colloid reactions were recorded on a JEOL ECZ 400S 400 MHz spectrometer. 1 H-PFG NMR diffusion measurements were performed in 5 mm tubes on a Bruker Avance III 400 MHz spectrometer at 300 K with a stimulated-echo sequence and default spoiler gradient (diffSte program, Bruker TopSpin). FT-IR spectroscopy. FT-IR measurements were carried out on a PerkinElmer Spectrum Two in attenuated total reflection (ATR) geometry equipped with a diamond crystal.
Super critical CO 2 activation. Super critical CO 2 activation was performed on a Leica EM CPD300 critical point dryer. Prior to super critical CO 2 activation, the COF samples were soaked in methanol.
Transmission electron microscopy. HRTEM characterizations were performed on an image-side spherical-aberration-corrected FEI Titan 80-300 operated under 300 kV. In order to reduce electron irradiation damage, low-dose conditions have been applied. SAED patterns were recorded with a typical dose of c.a. 2 e -/Å 2 , whereas the total dose for HRTEM imaging was limited to c.a. 70 e -/Å 2 . Sorption Measurements. N 2 sorption were measured with a Quantachrome Instruments Autosorb iQ 3 at 77 K. The pore size distribution (PSD) was determined with the calculation model of N 2 at 77 K on carbon using the QSDFT (cylindrical pores, adsorption branch) in ASiQwin software v 3.01. The COF samples were activated in high vacuum at 120 °C for 12 h before measurement. A suitable pressure region (P/P 0 = 0.050.2) was chosen for the BET surface area determination.
Grazing-incidence wide angle X-ray scattering. Two-dimensional grazing-incidence wide angle X-ray scattering (GIWAXS) data were recorded with an Anton Paar SAXSpoint 2.0 system equipped with a Primux 100 micro Cu K α source and a Dectris EIGER R 1M 2D detector. The COF films were positioned at a sample-detector distance of 140 mm and were measured with an incidence angle of 0.2°.
Ellipsometry. The complex refractive index of the TPB-MeOTP-NP film was obtained from ellipsometry measurements at room temperature (Sopra PS-1000 SAM).
Dynamic Light Scattering analysis. DLS measurements were carried out on a Malvern Zetasizer Nano ZS. The COF colloid was diluted with pure acetonitrile prior to DLS measurements.
Cyclic voltammetry measurements. The cyclic voltammetry measurements for determining HOMO and LUMO energy levels of P3HT and TPB-MeOTP COF were carried out on either an Autolab potentiostat or a WaveDriver 200 EIS Bipotentiostat. A three-electrode configuration was employed, with Ag/Ag + as reference electrode, Pt wire as counter electrode, and glassy carbon or carbon paper as working electrode. Anhydrous acetonitrile, containing 0.1 M tetrabutylammonium hexafluorophosphate, was used as supporting electrolyte. All the measurements were performed with the scan rate of 50 mV s 1 , under Ar atmosphere. The potential was calibrated against ferrocene/ferrocenium redox couple (−5.10 eV versus vacuum level).
UV-vis spectroscopy. UV-vis spectra of P3HT and TPB-MeOTP films were recorded on a Cary 60 UV-Vis Spectrophotometer.
Powder X-ray diffraction. Powder X-ray diffraction (PXRD) patterns were recorded in Debye-Scherrer geometry at room temperature, on a Stoe Stadi P diffractometer (Cu-K α1 ) equipped with a Ge(111) primary monochromator. All the samples were sealed in 1.0 mm glass capillaries, and measured with spinning for improving particle statistics.
Structure modeling. The modeling of TPB-MeOTP COFs, as well as the Pawley refinement of the COF structures, was performed with BIOVA Materials Studio 2017 (17.1.0.48. Copyright © 2016 Dassault Systèmes) suite. The geometry and the unit cell were optimized using force fields (Forcite, universal force fields with Ewald electrostatic and van der Waals summations method). To obtain the FWHM values of 100 peaks, the experimental PXRD profiles were fitted with a Gaussian function for 100 diffraction and a cubic polynomial function for subtracting the background. Representative fittings are shown in Supplementary Fig. 18. TPB-MeOTP film fabrication. TPB-MeOTP-NP and TPB-MeOTP-NS inks used for spin-coating were prepared by concentrating TPB-MeOTP-NP or TPB-MeOTP-NS reaction solution with a Heidolph rotary evaporator, under 115 mbar at 35 °C. No particle agglomeration or obvious crystallinity change was observed during this process. The mass concentration of the inks was 4.5 mg mL 1 and the digital images were displayed in Fig. 1. TPB-MeOTP-NP and TPB-MeOTP-NS were typically deposited on FTO substrates or other substrates by spin-coating the inks at 1500 rpm for 60 s (unless specifically stated otherwise). Multi coating cycles were performed with the same procedure. The resulting films were then post-annealed at 80 °C for 15 min in the air.
Photoelectrode fabrication. The FTO-coated glass substrates (Sigma-Aldrich, ~7 Ω sq 1 ) were subjected to a sequential sonication with deionized water (20 min, 5 times), acetone (20 min) and isopropanol (20 min), followed by drying with compressed air. CuSCN nanowire electrodeposition was carried out following to the procedure in the literature. 1 The precursor solution was prepared by first stirring the mixture of CuSO 4 (12 mM) and EDTA (12 mM) in deionized water for 10 min, and then adding KSCN (12 mM) and stir the resulting solution for overnight. The electrodeposition was performed using a three-electrode configuration with FTO substrate as working electrode, Pt as counter electrode and Ag/AgCl as reference electrode. The deposition potential was 0.3 V vs Ag/AgCl and the deposition time was 60 s. P3HT layer was prepared by spin coating 1 mg mL 1 P3HT solution in chloroform at 1500 rpm for 60 s, followed by a thermal annealing at 80 °C for 10 min in the air. Due to the hydrophobic surfaces of P3HT film, the first coating of TPB-MeOTP-NP or TPB-MeOTP-NS was at 6500 rpm, which is useful for achieving smooth films in the following process. This high spin-rate coating was not counted in the cycles as it only provides a low particle coverage on P3HT surface. TPB-MeOTP-NP or TPB-MeOTP-NS was then deposited as described above. A diluted SnO 2 solution (2.5 wt%) was used for SnO 2 layer fabrication 2 , prepared by adding water in the pristine SnO 2 colloid (15 wt%). The diluted SnO 2 was spin-coated at 1500 rpm for 60 s onto TPB-MeOTP-NP. The resulting films were transferred into a vacuum oven at 60 °C and annealed for 30 min. Pt catalyst overlayer was prepared by adding 16 µL Nafion 117 solution in 1 mL Pt nanoparticle dispersion as binder, and then spin-coated the solution onto COF films or SnO 2 at 1500 rom for 60 s. After the coating, the films were annealed at 80 °C for 15 min to remove the solvent residual.
Photoelectrochemical measurement. Photoelectrochemical measurements were carried out in a conventional three-electrode configuration powered by either an Autolab potentiostat or a WaveDriver 200 EIS Bipotentiostat. The COF photocathode was used as the working electrode with an active area of 0.283 cm 2 , and an Ag/AgCl (saturated KCl) electrode and a Pt wire were employed as reference electrode and counter electrode, respectively. Simulated 1 sun illumination (AM 1.5G, 100 mW cm 2 ) was provided by a Sciencetech solar simulator (XLH-E-500X) and calibrated by a Thorlabs photodiode (PM100D). PEC performance evaluation was performed by illuminating from the substrate side. Electrolyte (0.5 M) was prepared by dissolving Eu(NO 3 ) 3 or NaH 2 PO 4 in Nano-pure system purified water with the resistivity of 18.2 MΩ cm. The electrolyte pH was determined by a pH meter, calibrated with standard pH buffers. All experiments were carried out at ambient temperature and electrode potentials were converted to RHE scale using E RHE = E Ag/AgCl + 0.059 × pH + 0.197. Hydrogen evolution photocathodes was measured under Ar atmosphere, and the electrolyte was purged with Ar for 15 min prior to the measurements.

Solar hydrogen evolution detection.
A gas-tight single compartment polyether ether ketone (PEEK) electrochemical cell with 3-electrode configuration was used for gas chromatography (GC) measurements. Carrier gas Helium (99.9999%, Carbagas) was continuously infused into the cell with the flow rate of 10 mL min 1 (controlled by a digital thermal mass flow controller, Bronkhorst High-Tech, F201CV). The COF photocathode was illuminated from the substrate side with an applied potential of +0.7 V RHE for solar hydrogen production. A Newport LCS-100 solar simulator with AM1.5G filter was used as light source. Gas product was separated by a micropacked ShinCarbon column (Restek) and was analyzed by a pulse discharge detector (PDD, Vici) in an online GC (Trace ULTRA, Thermo). The control GC test without performing PEC measurement was taken after purging the electrochemical cell with Helium for 3 hours. The hydrogen retention time was calibrated using standard calibration gas (Carbagas AG), as shown in Supplementary Fig. 55. The COF photocathode with the working area of 0.6 cm 2 was operated at +0.7 V RHE and illuminated for 1697 s (a 10 s light chopping was performed at 14 s). The gas in headspace was sampled and analyzed for three times (Supplementary Fig. 54) to determine the hydrogen evolution from the COF photocathode.

Procedures for COF particle Synthesis
General procedures for COF synthesis. COF colloids including TPB-MeOTP, TPB-MeTP, TPB-TP were synthesized via a modified reported procedure. 3 In brief, MeOTP (or other terephthalaldehyde linkers) was firstly dissolved in dichloromethane and acetonitrile mixture. TAPB, with the molar ratio of 1:1 between amine groups in TAPB and aldehyde groups in terephthalaldehyde linkers, was then added in the solution. Sc(OTf) 3 with the amount of 0.08 equivalent of amine groups in TAPB, dissolved in acetonitrile, was injected into the reaction mixture via a syringe in 5 min. All the colloid reactions were performed with stirring at room temperature. The representative TPB-MeOTP-NP and TPB-MeOTP-NS were synthesized with [TAPB] of 1.97 mM and 5.02 mM, respectively. Solvothermally synthesized TPB-MeOTP was prepared according to the reported procedure 4 .
TPB-MeOTP-NP. 2,5-dimethoxyterephthalaldehyde (MeOTP, 17.2 mg, 88.6 μmol) and dichloromethane (1 mL) were placed in a Biotage 20 mL high precision glass vial and sonicated for 10 s. Acetonitrile (27 mL) was added in the vial, and the resulting solution was stirred at room temperature until MeOTP was dissolved. Then 1,3,5-tris(4-aminophenyl)benzene (TAPB, 20.8 mg, 59.2 μmol) was added in the vial. The vial was sealed with a cap and stirred at room temperature. Sc(OTf) 3 (7 mg, 14.2 μmol), 0.08 equivalent per amine functional group in TAPB, was dissolved in 2 mL acetonitrile and injected in the reaction tube within 5 min. The reaction was stirred at room temperature for 20 hours. For the reaction kinetic study, the reaction time was specified and controlled by a timer. The resulting particle solution was directly used for DLS and SEM characterization after dilution. To collect the particle powder for PXRD, BET, TEM and FT-IR measurements, 1 M NaCl aqueous solution (200 μL) was added in the vial to precipitate out the particles. The precipitates were collected with a filter paper, washed with acetone, dimethylformamide, chloroform and methanol, and subjected to Soxhlet extraction with methanol for 12 h. The final product (30.6 mg, yield: 87.9%) was obtained after activating with supercritical CO 2 . In the whole workup process, the precipitates remained wet prior to the super critical CO 2 activation.

TPB-MeOTP, TPB-MeTP and TPB-TP colloid reactions with varying [TAPB].
Following the procedure of TPB-MeOTP-NP, [TAPB] was varied from 5.02 mM to 0.52 mM, while the ratio of amine groups in TAPB and aldehyde groups in terephthalaldehyde linkers and Sc(OTf) 3 amount were fixed at 1:1 and 0.08 equivalent, respectively. After the reaction, the colloids were precipitated out with 1 M NaCl aqueous solution. The precipitates were collected via filtration, washed with acetone, dimethylformamide, chloroform and methanol, and activated with supercritical CO 2 . Soxhlet purification was not performed in this series of synthesis, as it only provides a minor sharpness for 100 peak in PXRD and does not affect the comparison. The amount of the chemicals used for the colloid reactions are summarized in Supplementary  Table 1. The reaction yield of the reactions was summarized in Supplementary Fig. 19. TPB-HTP reaction. Following the procedure of TPB-MeOTP-NP, TPB-HTP was synthesized under [TAPB] of 2.81 mM and 1.97 mM, with the ratio of amine groups in TAPB and aldehyde groups in HTP and Sc(OTf) 3 amount fixed at 1:1 and 0.08 equivalent, respectively. Higher [TAPB] condition cannot be performed due to the limited solubility of HTP in acetonitrile. Different from TPB-MeOTP, TPB-MeTP and TPB-TP, the TPB-HTP product was precipitated out from the solution and no colloid solution was obtained. The obtained TPB-HTP has a very low crystallinity, suggested by the nearly invisible 100 diffraction peak (Supplementary Fig. 15).

In-situ 1 H-NMR characterizations
The molar concentration of the linkers for in-situ 1 H-NMR characterizations followed with the TPB-MeOTP-NP synthesis. CDCl 3 was used as the co-solvent instead of CD 2 Cl 2 , due to its lower cost and very similar property with CD 2 Cl 2 . Here, the in-situ 1 H-NMR characterizations of TPB-MeOTP-NP was taken as an example to describe the procedure.

Solvothermally synthesized TPB-MeOTP
A Biotage 2 mL high precision glass vial was charged with TAPB (35.1 mg, 100 μmol), MeOTP (29.1 mg, 150 μmol), acetic acid (6 M, 125 μL), dichlorobenzene (625 μL) and butanol (625 μL), and degassed by purging Ar for 10 min. Then the vial was heated at 120 °C for three days. After cooling to room temperature, the product was collected with a filter paper, washed with acetone, dimethylformamide, chloroform and methanol, and purified with methanol in a Soxhlet extractor for 12 h. After activating with supercritical CO 2 , 52.2 mg of TPB-MeOTP was obtained, offering a reaction yield of 88.8%.

Ellipsometry modeling
The complex refractive index of the TPB-MeOTP-NP film, Fig. 5f , was obtained ( ( ) = ( ) + ( )) from ellipsometry measurements at room temperature (Sopra PS-1000 SAM). We employed a genetic algorithm to fit the experimental data assuming a Forouhi-Bloomer model for three oscillators [5][6] : where and here E is the photon energy, E g is the bandgap energy, and E j , f j , and  j are the position, strength, and width of one oscillator. In order to make the method more accurate and reliable, two films with different thicknesses at three different incident angle (60 o , 65 o and 70 o ) measurements were fitted simultaneously.
A dielectric function p models the porosity of the film assuming a Bruggeman effective medium approximation: where p is the porosity, i.e. the air to COF ratio, of the layer. A layer of arbitrary thickness (d r , 0 nm -40 nm and a porosity of p r ) and dielectric function r models the surface roughness of the film assuming also Bruggeman effective medium approximation. To reduce the impact of randomness on the initial fitting parameters, some of the parameters were appropriately bounded according to the experimental values of the layer thickness (from SEM analysis) and the bandgap (from Tauc plot). Dielectric function parameters of the best fitting was summarized in Supplementary Table 4. Once the refractive index is obtained, we used it to calculate the electric field intensity distribution and absorption maps following the transfer matrix method (TMM) and the following equation 7 for a sample immersed in water: where the j refers to each layer in the sample and λ is the wavelength. The integral extends over the volume occupied by each medium, which can be reduced to one dimension (x j ) due to the symmetry of the system. 3.930

Solubility estimation
The solubility of TP, MeOTP, MeTP and HTP in acetonitrile at room temperature was estimated by adding the minimal volume of acetonitrile in a known amount of linker and making the linker fully dissolve. Sonication and stirring were performed for assisting the dissolution. No heating treatment was used, as the colloid reaction was performed at room temperature.

PFG NMR characterization
The pulsed field gradient NMR technique (PFG NMR) was applied to determine the diffusion coefficient The D of molecules in solution can be described by the Stokes-Einstein equation (1), containing the Boltzmann constant (k B ), the temperature (T), the viscosity (η) and the hydrodynamic radius of the diffusing species. The diffusion is inversely proportional to the hydrodynamic radius if the temperature and solution viscosity remain constant. As a consequence, changes in D at different concentrations can be used to study the self-assembly behavior of the solute, as the self-assembly of multiple molecules increases the apparent hydrodynamic radius 8-10 of the diffusing species.
= 6 (1) Tetramethylsilane was added as an internal standard for the solvent viscosity, which remained constant at the measured solute concentrations. The measurements were performed in 5 mm tubes on a Bruker Avance III 400 MHz spectrometer at 300 K with a stimulated-echo sequence 11 and default spoiler gradient (diffSte program, Bruker TopSpin). A gradient pulse with a length δ = 1 ms (opt shape) and a diffusion time Δ = 50 ms were applied. The gradient was varied linearly in 16 steps between 2.33 and 46.55 Gs/cm. Data processing was performed with the Bruker TopSpin 3.5 Dynamics module using automated peak picking, integration and fitting of the signal area S vs. gradient B according to the Stejskal-Tanner equation (Eq. 2). Experiments were performed multiple times with an appropriate number of scans for the individual sample concentration (standard deviation is shown as error bars).
The resulting D of TP and MeOTP at different concentrations is shown in Supplementary Figure 30. The diffusion coefficient of Tp and MeOTP in deuterated acetonitrile increases with decreasing concentration, indicating that both building blocks form molecular aggregates in solution. However, TP molecules interact less strongly compared to MeOTP, visible from the early increase of the diffusion coefficient with decreasing concentration, which correlates well with the fast crystallization rate of TPB-MeOTP as well as the preferred particle growth of nanoplates along the interlayer direction. We can infer that the crystallite formation in this system is directed by the template effect of the self-assembly in solution.

Supplementary Figures
Supplementary Figure 1 . The natural logarithm of aldehyde proton peak area, ln(peak area), = 0 is plotted against the reaction time (t). The aldehyde consumption rate of each reaction is obtained by performing a linear fitting between ln(peak area) and reaction time (t). Figure 30. Diffusion coefficients of TP and MeOTP at different concentrations in acetonitrile-d 3 . The diffusion coefficient (D) value is correlated to the hydrodynamic volume of the diffusing species in the solution. The lower D in concentrated solutions is due to the self-assembly of molecules. 9 The data shows that both TP and MeOTP self-assemble at high concentration (12 mM). Nevertheless, the D of TP molecules starts to increase from ~6 mM, and in contrast, the D increase of MeOTP molecules occurs at a lower concentration (~1.5 mM). The results indicate that MeOTP has a stronger tendency to self-assemble in solution, compared to TP. The concentration of TAPB in the corresponding reaction condition is shown as a reference. Nanoplates are achieved at low concentrations, where the self-assembly of MeOTP assists the particle growth along the interlayer interaction direction. In comparison, nanospheres are formed at high concentrations of MeOTP. This is correlated to higher concentrations, as the agglomeration of TPB-MeOTP crystallites could occur during the crystallite growth. Nevertheless, P3HT layer exhibits anodic current at +0.79 V RHE for FTO/ P3HT and +0.76 V RHE for FTO/CuSCN/P3HT even under dark condition, very likely due to the oxidation of P3HT as the HOMO level locates at about +0.81 V RHE . The P3HT/COF-NP heterojunction positively shifted the V on in both cases with and without CuSCN nanowires. All the CA tests were collected at +0.7 V RHE in this work, since the photocurrent contribution from P3HT is negligible at this potential, as indicated in Fig. 6d in the main text. Introducing CuSCN nanowires reduces the dark current density of FTO/P3HT/COF-NP (16.9 µA cm 2 ) at +0.20 V RHE to 10.9 µA cm 2 , and increases the J ph from 65.2 µA cm 2 to 81.6 µA cm 2 , suggesting a reduced recombination for hole extraction.